Welcome Jeanna,You don't always have to do an excursion in every port...I would do some research at each port and see what interests you. You can look through the Carnival web-site and see what they offer.

If you haven't already been to Grand Cayman, I'd suggest snorkeling with the sting rays at Sting Ray City, a really great time. If you do enjoy snorkeling, Belize has some great tours and also the rafting...

At Cozumel, you can take a cab over to Chankanaab Park and spend the day, great beach and they offer the swim with dolphins which is also a lot of fun....